The use of communication cables which include a plurality of optical fibers is rapidly expanding. An optical fiber cable may comprise a plurality of glass fibers each of which is protected by at least one layer of a coating material. The optical fibers may be assembled into units in which the fibers are held together by binder ribbons or tubes to provide a core. Another optical fiber cable core includes a ribbon type optical fiber arrangement in which a plurality, such as twelve fibers for example, are arrayed together side by side. A plurality of these fiber ribbons may be stacked to obtain a high fiber count cable. The core is enclosed by a plastic tube and a plastic jacket. Ribbon type cable in which a relatively large number of readily accessible optical fibers may be packaged appears to be ideally suited for fiber-in-the-loop use.
Whatever the structure of a cable, there must be provisions for splicing transmission media at an end of a given length of cable to corresponding transmission media at an adjacent end of another length of cable. In wire-like metallic conductor communication practice, it is conventional to use a splice closure, within which all conductors are spliced, wrapped and stored and protected environmentally.
During the splicing of metallic conductors, it is customary to bend sharply the conductors, to provide access to other connections. The physical nature of glass optical fibers forecloses the adoption of splicing techniques which are used with metallic conductors within such a splice closure. Because of their small size and relative fragility, special considerations must be given to the handling of optical fibers in closures. Transmission capabilities may be impaired if an optical fiber is bent beyond an allowable bending radius, the point at which light no longer is totally contained in the core of the fiber. Furthermore, expected lives of the fibers will be reduced if bent to less than the minimum bending radius. Generally, the radius to which the optical fiber can be bent without affecting orderly transmission is substantially greater than that radius at which the optical fiber will break. Whereas glass and silica, the materials used to make optical fibers, are in some respects stronger than steel, optical fibers normally do not possess this potential strength because of microscopic surface fractures, which are vulnerable to stress and spread, causing the fiber to break easily.
It should be clear that, an optical fiber cable does not lend itself to the splicing practices of wire-like communication conductors. The individual glass fibers cannot just be twisted, tied, wrapped and moved into a splice closure, in anything like the manner of wire-like metallic conductor cables. These small diameter glass fibers cannot be crimped or bent at small angles, without breakage. Inasmuch as glass fibers have memory and tend to return to a straight-line orientation, placement in a splice closure becomes somewhat difficult. Moreover, the interconnection of optical fibers is a precision operation which is somewhat difficult to perform within a manhole, or at pole-suspension elevation, for example. These problems are particularly acute in multifiber cables where individual optical fibers must be spliced in a manner which allows repairs and rearrangements to be made in the future.
In addition, fiber slack normally must be provided adjacent to splices. When splicing optical fibers by mechanical means or by fusion, it becomes necessary to provide enough slack fiber so that the fiber can be pulled out of a closure for the preparation of fiber ends and the joining together. This requires at least about 0.5 meter of fiber from each cable to be stored in the splice closure when the closure is sealed, that is when the splicing has been completed. For a multifiber cable there must be a method of storing this slack, of protecting the splices and of keeping the fibers together in an orderly manner. The splices should be easily accessible to facilitate the rearrangement of the optical fibers and splices. The need to store the slack further complicates the problem of providing a suitable optical fiber closure.
Furthermore, there are a number of different kinds of splicing arrangements which are used commercially. Desirably, a closure should be capable of accommodating at least the more popular of these splicing arrangements.
Also, there is a need for a closure which is particularly suited in the fiber-in-the-loop market and to splice relatively small count optical fiber cables some of which are referred to as drop cables. For such a use, what is sought after is a closure that is relatively inexpensive to serve this very large market. Also, desirably, the sought after closure is relatively small in size yet able to accommodate a relatively large number of splices.
As might be expected, fiber splice closures are available in the prior art. Some of these prior art closures have shortcomings. Typically, they have been somewhat complex, difficult to use and difficult to access. Some prior art splice closures have included organizers which have tended to place higher than desired stresses on the optical fibers, resulting in fiber damage. In addition, these prior art closures often have failed to provide simple to use, positive means for routing the optical fibers in an effective manner and for storing slack.
For example, a splice closure with a central transverse bulkhead has been used. Individual fibers are spliced and are attached to the bulkhead for support. A disadvantage of this approach is the lack of facilities for the storage of slack in the fibers. In other splicing arrangements, all the optical fibers in a cable are looped within the same retainer or fiber slack is stored on spools. In either case, identification, repair or splice work of individual fibers is extremely difficult without a major rearrangement within the splice closure. This is undesirable because the transmission capability in active fibers can be affected as they are moved.
In another closure of the prior art, there is provided a device having a modular construction which is suitable for installation in standard splice closures. The device comprises a plurality of tray-like members each adapted to retain and store at least one fiber. The device provides access to the individual fibers contained in the trays. The trays are stacked one on top of the other, and each is hinged separately at one end thereof to a carrier, thus allowing them to move relative to one another like bound pages. Each tray-like support has a width which is adequate to provide the minimum bending radius specified for that fiber.
In another prior art optical fiber cable closure, optical fiber transitions with a controlled bend radius are anchored from each cable to a hinged organizer tray. This arrangement provides ready access to in-service optical fibers without the risk of inadvertent bending of the fibers. However, the arrangement of optical fibers in a cable to different trays is somewhat cumbersome to carry out and there appears to be a lack of protection for the fibers in the transition from the cables to the trays. This problem has been solved by the arrangement shown in U.S. Pat. No. 4,927,227, which issued on Apr. 22, 1990 in the names of W. H. Bensel, et al. Therein, a support member includes a base for supporting an optical fiber breakout and a plurality of splice trays. The breakout allows a user to separate fibers into groups before they are routed to ones of the trays.
In still another closure, a tubular cover having a closed end and an open end is adapted to receive and be sealed to a cable termination assembly. The cable termination assembly includes cable entry facilities through which the cables to be spliced are routed. A support member extends from the cable entry facilities and has a free end disposed adjacent to the closed end of the cover. The support member includes a support base for supporting an optical fiber breakout and a plurality of optical fiber splice trays.
Mounted centrally of each tray is at least one organizing module each of which is capable of holding a plurality of optical fiber connective arrangements. Each module is such that it is capable of accommodating different kinds of connective arrangements such as, for example, fusion splices and cleave, sleeve, and leave splicing connectors. Each tray is capable of holding a plurality of organizing modules which may be added as needed. Although this last-described closure has enhanced storage capability both in number and in kind, which is ideal for high density applications, it is larger and has more storage capability than is needed for a market such as fiber-in-the-loop and for splicing small fiber count cables.
What the prior art seemingly lacks is an optical fiber cable closure which is relatively small in size and which is relatively inexpensive. The sought after closure should have suitable storage capability and also the capability to store different kinds of splicing arrangements.